French police investigating a series of rapes are perplexed after DNA evidence matched a set of twins - but they cannot tell which brother may be to blame.

With telling the difference between the twins' DNA extremely difficult and expensive, police in the southern city of Marseille have in the meantime charged both men, identified only as Elwin and Yohan, and are holding them without bail.

"It's a rather rare case for the alleged perpetrators to be identical twins," chief investigator Emmanuel Kiehl said.

The two men, both 24-year-old unemployed delivery drivers, deny any involvement in the rapes of six women between between September and January.

Police admit that without far more extensive tests it will be difficult to figure out which of the twins was possibly behind the attacks or whether both men were indeed involved.

Police tracked the twins down through video footage recorded on a bus and a mobile telephone allegedly taken from one of the victims and found in the brothers' possession.

The victims' mobile phones were taken in each of the attacks, which took place in the corridors of buildings and involved women aged 22 to 76.

Police said the victims were also able to identify the suspects, but not to tell them apart.

Mr Kiehl said DNA evidence was found at some of the crime scenes but that regular tests were incapable of differentiating between the twins.

The cost of extensive-enough tests would be "onerous", he said.

Local newspaper La Provence reported that police were told it could cost up to 1m euros (£845,000) for the necessary tests.

Only the smallest of differences exist in the DNAs of identical twins.

"For a normal analysis we compare 400 base pairs," a DNA expert told La Provence, adding that with twins: "We would be looking at billions."